A wide variety of electronic circuit applications employ differential measurement of two currents and some prescribed amount of rejection of common mode components of the currents. In some applications, the currents being measured may exist in a high DC voltage/current environment, while their information content is ultimately employed in a low voltage/current environment, with demanding requirements for accurate amplification and filtering. As a non-limiting example, various equipment employed by telecommunication service providers utilize subscriber line interface circuits (SLICs) to interface telecommunication signals with phone line wiring that may include substantial DC components.
As another example, fully balanced for differential transimpedance amplifiers may be employed in optical receiver systems, where a current-varying output signal of a photodetector is converted into a voltage signal to be processed by other circuitry. While amplifiers with a balanced load may provide a preferred solution for applications such as those listed above, other considerations such as component count, current dissipation, and the like may result in employment of hybrid circuits.
An amplifier with a balanced differential load offers distinct advantages over a single-ended amplifier or an amplifier with an unbalanced differential load such as a current mirror. Specifically, an amplifier having a balanced differential load significantly enhances noise immunity whether the noise originates from either the positive or negative supply or from a switching event that injects an equal signal into each side of the load. In an amplifier having a balanced differential load, the occurrence of noise will cause a signal which is common to both outputs and, therefore, ideally will not generate a differential signal which can be amplified by subsequent stages. A balanced differential load can be achieved either actively or passively, each method has its own caveats. The simplest method involves using a pair of matched resistors to create a passive differential load. Resistive differential loads tend to have the best matching and the best controlled impedance. The disadvantage is that there is a large IR voltage drop and a relatively large silicon area is needed in order to achieve appreciable gain. An active differential load is generally comprised of two transistors connected either as current sources or as diodes. When used as current sources, a separate amplifier is required in order to provide common-mode feedback (CMFB) to quiescently bias the transistors such that they function as current sources during steady-state operation. Aside from added circuitry and complexity, a CMFB loop may also degrade the stability of a system. In addition, the output impedance of an amplifier with a current source load is determined by either channel length modulation (MOSFETs) or base width modulation (BJTs), each of which can cause large variations in gain and pole locations of the amplifier because of their control difficulties. Normally diode loads have higher voltage loss than current sources and generally do not provide high impedances.
An amplifier with a balanced differential load may provide significantly improved noise immunity compared to a single-ended amplifier or an amplifier with an unbalanced load such as a current mirror.
Thus, it is with respect to these considerations and others there is disclosed a new type of balanced differential load having the advantageous attributes of resistive loading: well controlled impedance, improved matching and no need for CMFB, as well as the benefits of active loading with low voltage loss and smaller area requirements.